Many processes and catalysts are known for the preparation of homopolymeric or copolymeric olefins and other polymers. Ziegler-Natta catalyst compositions, developed in the 1950s, were found to be particularly useful in the preparation of polyolefins. These catalyst compositions comprise transition metal compounds such as titanium tetrachloride and an alkylaluminum (e.g., triethylaluminum) cocatalyst. The systems were found to be advantageous because of their high activity, and were largely consumed during polymerization.
Subsequent catalyst systems have been designed to provide more control over polymer structure and properties than could be achieved with Ziegler-Natta catalysts. These later catalysts have well-defined active sites and can be rationally designed to produce a specific polymer product, i.e., having predetermined structure and properties. Such catalysts include, for example, metal complexes known as “metallocenes.” The term “metallocene” was initially coined in the early 1950s to refer to dicyclopentadienyliron, or “ferrocene,” a structure in which an iron atom is contained between and associated with two parallel cyclopentadienyl groups. The term is now used to refer generally to organometallic complexes in which a metal atom (not necessarily iron) is coordinated to at least one cyclopentadienyl ring ligand. A. D. Horton, “Metallocene Catalysis: Polymers by Design,” Trends Polym. Sci. 2(5):158-166 (1994), provides an overview of metallocene catalysts and their advantages, and focuses on now-conventional complexes of Group IV transition metal complexes and cyclopentadienyl ligands (Cp2MX2, wherein Cp represents a cyclopentadienyl ligand, M is Zr, Hf or Ti, and X is Cl or CH3). Unfortunately, however, although metallocenes do provide significant advantages relative to the traditional Ziegler-Natta catalysts, the high cost and difficulties associated with heterogenization of metallocenes, as well as the oxophilic nature of the early transition metals, have limited the applicability of metallocenes as commercial polymerization catalysts.
Because polyolefins such as polyethylene and polypropylene are such important commercial polymers, there is an ongoing need for improved polymerization techniques and polymerization catalysts. Recently, researchers have developed new catalysts suitable for olefin polymerization that are complexes of late transition metals and substituted diimine ligands. Such catalysts are described, for example, in Bres et al., PCT Publication No. WO 98/49208, published Nov. 5, 1998. Other similar catalysts, comprised of diimine ligands and selected metals, are described in Bennett, PCT Publication No. WO 98/27174, published Jun. 25, 1998, and in Brookhart et al., PCT Publication No. WO 98/30612, published Jul. 16, 1998. While these catalysts have some advantages, they are lacking in several significant respects. Perhaps most importantly, the aforementioned catalysts are incapable of producing commodity polymers such as linear low density polyethylene (“LLDPE,” having a density of about 0.918 to 0.935 g/cm3) and isotactic polypropylene (“iPP”).
The present invention is thus addressed to the aforementioned need in the art, and provides novel compounds useful as polymerization catalysts, e.g., in the polymerization of olefins. The catalysts provide for numerous advantages relative to the polymerization catalysts of the prior art, in that they:                (1) are simple and cost-effective to synthesize;        (2) allow for exceptional control over the structure and properties of the polymeric product;        (3) are highly active polymerization catalysts;        (4) can be used in stereospecific polymerization to provide stereoregular polymers, including isotactic and syndiotactic polymers;        (5) enable preparation of commodity polymers such as linear low density polyethylene and isotactic polypropylene;        (6) can be used as either supported or homogeneous polymerization catalysts;        (7) are quite versatile and can be used in conjunction with a variety of monomer types; and        (8) can be used to catalyze reactions other than polymerization reactions, e.g., hydrogenation.The invention thus represents a significant advance in the field of catalysis, as prior to the development of the catalysts disclosed and claimed herein, only a few of the aforementioned advantages could be achieved with a single catalyst system.        